It is known to provide a control valve, arranged to control movement of a fuel injector valve needle relative to a seating, so as to control delivery of fuel from a fuel injector. In known embodiments, movement of a valve needle away from the seating typically permits fuel to flow from a delivery chamber, through an injector outlet, and into the engine cylinder or other combustion space.
Typically, a control valve includes a control valve member that is moveable between a first position, in which fuel under high pressure is able to flow into the control chamber, and a second position, in which the control chamber communicates with a low pressure fuel reservoir, such as a low pressure fuel drain. A surface associated with the valve needle is exposed to fuel pressure within the control chamber such that the pressure of fuel within the control chamber applies a force to the valve needle to urge the valve needle against its seating.
In order to commence injection, the valve is actuated such that the control valve member is moved into its second position, thereby causing fuel pressure within the control chamber to be reduced. The force urging the valve needle against its seating is therefore reduced and fuel pressure within the delivery chamber serves to lift the valve needle away from its seating to permit fuel to flow through the injector outlet. In order to to terminate injection, the valve is actuated such that the control valve member is moved into its first position, thereby permitting fuel under high pressure to flow into the control chamber. The force acting on the valve needle due to fuel pressure within the control chamber is therefore increased, causing the valve needle to be urged against its seating to terminate injection.
For optimal injector performance, it is desired to control the rate at which the valve needle of the injector lifts so as to provide a controlled increase in injection rate. However, it is also desired to terminate injection rapidly.
Such asymmetric control is typically achieved by providing a flow restriction in the control valve so that the rate of flow of fuel between the source of high pressure fuel and the control chamber is controlled. However, in this type of control valve unbalanced hydraulic forces are created as a result of the flow of fuel past the valve seating. These unbalanced forces act on the control valve member and can cause the control valve member to ‘stall’ between a first, non-injecting position and a second, injecting position, and this has a detrimental effect on injector performance. However, the use of this restriction decreases the rate at which the control chamber is pressurised, and therefore the rate at which the valve needle of the injector is urged against the needle seating to terminate injection. Furthermore, depressurisation of the control chamber can occur rapidly, giving rise to relatively fast needle lift. Such characteristics are not considered to provide optimal injector performance.
EP 1604104A describes a flow restriction that achieves asymmetric control. The restriction is provided in the control valve to control the rate of flow of fuel from the control chamber to the low pressure drain during transition of the control valve member from the first position to the second position. The flow restriction results in a slower decrease in pressure within the control chamber and, consequently, a slower speed at which the valve needle of the injector lifts away from the needle seating. At the same time the benefits of rapid termination of the injection can be achieved because the flow rate to terminate injection is not hindered by the restriction. The valve movement therefore has an asymmetry between its rate of opening movement and its rate of closing movement. Accordingly, this control valve provides movement damping for a controlled increase in injection rate. The control valve is also pressure balanced in both the first and second positions.
One of the control valves disclosed in EP 1604104A has a flow restriction that passes between the outer surface of the control valve member and the internal surface of the bore within which the control valve member moves. Of the various control valves described, this embodiment is the simplest and cheapest to manufacture, because it neither has an additional drilling through the control valve member, nor an insert in the bore of the housing, such as a sleeve or a balance piston, that defines the flow path restriction. However, a problem with this control valve is that it experiences the unbalanced forces, as described previously, during transition between the first and second positions with a resulting detriment in performance. It has been found that when the width of the valve seating is increased the unbalanced forces become more significant, compromising the performance of the control valve, whereas reducing the width of the valve seating compromises endurance.
GB 2041170A teaches the use of a control valve comprising a valve member having a restriction in a passage leading to a low pressure fuel drain. However, the control valve comprises a spool valve that has only two ports, being in communication with an injection pump when the control valve is in a first position and in communication with the low pressure drain when the control valve is in a second position.